(1) Field of the Invention
The present invention relates to the synthesis of aliphatic polyester-grafted polysaccharides prepared by in situ polymerization of a cyclic ester monomer in the presence of a polysaccharide and an organometallic catalyst initiator, wherein the ring-opening polymerization is conducted via bulk polymerization of the ester monomer, preferably in the absence of solvents. The present invention preferably relates to the synthesis of aliphatic polyester-grafted starch-like polysaccharides. The present invention further relates to novel compositions based on blends and composites of the aliphatic polyester-grafted polysaccharides with other thermoplastic polymers, and other additives commonly used in the plastics industry, in which there is good interfacial adhesion between the component phases. Durable and cost-effective materials are produced. The compositions are suitable for use in a number of applications including biodegradable moldings, sheets, films, or foam structures and are processed by any of the melt processing methods employed in the plastics industry.
(2) Description of Related Art
In the last decade, considerable effort has gone into the development of biodegradable polymers, including polymer blends and composites, using starch. The melt processing characteristics and mechanical properties of unmodified starch polymers are very poor compared to typical synthetic polymers. Some of the problems associated with starch based compositions include thermal decomposition of starch before melting, water absorption, and poor mechanical properties. Physical or chemical modifications of the starch molecule or granule is a viable alternative to solving some of these problems. Physical modifications include coating the starch granules with hydrophobic sizing agents similar to those used in the paper industry, like rosin and silanes or physically coating the end product with hydrophobic materials like low molecular weight waxes, and natural resins (zein, rosin, shellac etc.) and higher molecular weight non-polar polymers. Crosslinking of the starch granules is another physical modification route to improving the hydrophobicity of starch-based materials. A physico-thermal route to improving the melt processability of starch is by the use of an external plasticizer to solvate the starch granules and enhance the room temperature flexibility of the final product. In a typical plasticized system with starch, the diffusion of plasticizer out of the product when exposed to low humidity conditions and diffusion of water in to the product under high humidity conditions is an inevitable result. This causes embrittlement of the product due to loss of plasticizer (low humidity) and problems associated with retention of product shape, texture, and form due to excess absorbed water (high humidity). These effects are detrimental particularly when water is used as a plasticizer, but are prevalent even in non-water based starch formulations that incorporate hygroscopic plasticizers. In general, due to the poor durability of plasticized starch upon exposure to different environments (due to its hydrophilic nature) there is very little commercial use of plasticized native or unmodified starch by itself. Early patents in this field relate to the extrusion of amylose primarily using water as a plasticizer. In addition, poly(vinyl alcohol), glycerol and other related materials were also incorporated into the system. More recently, extruded compositions using water and urea as destructurizing agents for starch and the simple blends of these with other polymers were discussed in PCT Int. Appl. WO 92/19680 (Novamont S.p.a.). Due to the poor compatibility between various components of the blend, compositions containing significant amounts of starch exhibited poor mechanical properties. There is a great need to enhance the interfacial compatibility and other properties in systems using starch.
Chemical modification of starch includes grafting reactions and non-degradative substitution of the hydroxyls on the starch with functional groups such as esters, ethers, isocyanates and the like. A number of starch derivatives with varying degrees of substitution have been prepared, primarily for food applications and more recently for structural applications. Starch graft copolymers produced from various vinyl monomers, including styrene, methyl methacrylate, methyl acrylate, and butyl acrylate, containing about 50% starch by weight, have been prepared by a solution process in which the starch grafting was initiated by radiation in the case of styrene and by cerium ion in the case of other monomers (Bagley, Fanta et. al., Polymer Engineering and Science, 17 (5), p. 311, (1977)). These compositions were extruded directly without addition of plasticizer or homopolymer to give useful products. However, the reaction times for polymerizations were on the order of hours. Starch acrylamide copolymers were prepared by Ce.sup.4+ initiated grafting reactions in solution (Pledger, Young et. al., J. Macromol. Sci.-Chem., A22(4), p. 415, (1986)). Starch-acrylonitrile copolymers, also obtained by Ce.sup.4+ initiated grafting reactions in solution are also known and suffer from the same processing drawbacks. Anionic polymerization of ethylene oxide on starch has also been reported (Tahan and Zilkha, Journal of Polymer Science: A-1, 7, p. 1815 (1969)). Reactive extrusion of starch graft copolymers using starch macroradicals generated by shear inside an extruder in the presence of vinylic monomers and/or polymers leading to low levels of grafting was studied by Chinnaswamy and Hanna (Starch/Starke, 43 (10), p. 396 (1991)).
Polysaccharide grafted polymer derivatives are well known to those skilled in the art. Such derivatives are described in U.S. Pat. Nos. 3,321,422 to Houff et al (unsaturated monomers); 4,079,025 to Young et al (unsaturated monomers); 4,454,268 to Otey et al (acrylic monomers); 4,663,388 to Douglass et al (unsaturated monomers); 5,191,016 and 5,268,422 to Yalpani (polyhydroxyalkanoates); 5,217,803 and 5,254,607 to McBride et al (unsaturated monomers); and 5,286,770 to Bastioli et al (unsaturated monomers).
Biodegradable polymers of lactones and other cyclic ester monomers are well known as illustrated by U.S. Pat. No. 5,321,088 to Schwab. What is needed are polysaccharide graft-polymers of the cyclic ester monomers, in which the grafting leads to good processing and mechanical properties.
There is a need for novel grafted polysaccharides which are biodegradable and which contribute to making the polysaccharide compatible with other polymers. Such aliphatic polyester-grafted starch material can be used as compatibilizers to promote good interfacial adhesion.